In the present connection high viscosity fluids mean fluids with a viscosity of approx. 1 Pa.sec. and above.
Most industrially used viscometers are rotation viscometers built in such a way that the viscosity of highly viscous fluids, such as offset ink, is only measurable at low shear rates. This is partially due to the often distinct visco-elastic properties of the fluids, and partially due to insufficient motor power. A further reason is that due to the heat generated during the measuring it is impossible to obtain a constant temperature of the fluid the viscosity of which is to be measured.
Examples of rotation viscometers for determining a flow curve are disclosed in DE-OS 2.310.461 and DE-AS 2.314.671.
It is also known to use viscosity measuring devices where a metal rod is moved downwards through a fixed ring and the fluid the viscosity of which is to be measured is applied to the inner side of said ring. The metal rod is subjected to different loads to determine correlated values for shear rates and shear stresses, cf. for instance DE-AS 2.754.075. The disadvantage when using such a rod viscometer is that in practice it is unavoidable for the rod to occasionally fall askew resulting in incorrect measurements. Moreover such a device can only determine one or a few measuring points on the flow curve by each measurement. Such a device is further disadvantageous in that the temperature is either impossible or very difficult to adjust and that the height of the device is necessarily at least twice the length of the rod.
GB specification no. 2.120.793 A discloses a viscometer for measuring the rheological properties of Newtonian fluids and Bingham plastics, where a control piston passes through a hollow cylinder comprising the fluid the viscosity of which is to be measured. The shear rate of the fluid in the gap between the piston and the cylinder, however, varies greatly between zero in the middle of the gap and a maximum at the limiting surfaces. Thus such a viscometer is in reality a specially constructed capillary viscometer and is thus unable to determine a flow curve where each measuring point derives from a well-defined, constant shear rate.